This invention relates to rotary valves for internal-combustion engines and, more particularly to apparatus to obtain variable timing of intake opening, variable intake opening duration and variable timing of intake closing of rotary valves.
The timing of the opening of the intake and exhause valves of an internal combustion engine is inflexible once established by the design of the camshaft in a conventional poppet valve engine, or by the design of the head ports and circumferential valve body openings in a typical rotary valve engine. The appetite of an engine for an intake charge, however, is quite different at high RPM and high load than it is at low speed and light load, or at idle and the effect of gas momentum at these dissimilar operating modes has a significant effect on performance, fuel economy, and emissions.
At high engine speeds and moderate to heavy loads, for example, a lengthy intake valve opening duration is required to permit efficient breathing and maximum power. Early opening of the valve [typically 10-25 degrees BTDC] increases the length of time the valve is wide open during the early part of the stroke and late closing [typically 45-65 degrees ABDC] allows the charge momentum to continue filling the cylinder even though the piston is moving upward on the compression stroke. During high load operation the exhaust gases exit the cylinder with such intensity that a relative vacuum can occur at the end of the exhaust stroke. An early intake valve opening can then be advantageous in assisting cylinder scavenging as well as in obtaining increased volumetric efficiency.
During low engine speed-light load operation the situation changes and the pressure of the exhaust gases in the cylinder exceeds the pressure in the intake manifold so that early valve opening results in exhaust gases entering the intake system, diluting the fresh mixture and reducing combustion efficiency. This is particularly significant at idle since the fuel system must compensate for this dilution with extra rich mixture which increases fuel consumption and also the probability of increased emissions.
Late closing of the intake valve at speeds and loads [including low to medium speed and light to medium loads] where charge momentum does not at least counteract the piston's upward push on the intake charge, results in already inducted intake charge being pushed back into the intake system and reduces the compression of the intake charge and the engine efficiency.
Inflexible valve timing, therefore, forces the engine designer to compromise in areas of performance, fuel economy and emissions since these areas are linked to and partially dependent on valve timing and improvements in one area usually result in deterioration in at least one of the others.
Examples of prior attempts to obtain variable valve timing are included in patents to Tischler and Guenther.
Tischler, in U.S. Pat. No. 3,993,036 describes a variable timing valve which is responsive to engine speed. It retards the time of valve closing without affecting the timing of valve opening. This design provides greater than desirable valve opening during high engine speed-light load operating modes such as coasting.
Guenther, in U.S. Pat. Nos. 3,948,227 and 4,036,184 describes externally excited, load responsive apparatus to obtain variable valve timing in valves driven at 1/4 engine speed. The apparatus is not applicable to valves operating at 1/2 engine speed and precludes obtaining the advantages of such designs. In addition, the timing of the described valve being dependent on throttle opening, or vacuum will provide poorer than optimum timing and valve opening duration when the engine is accelerating from low speed. Consequently, poorer performance and reduced combustion efficiency are obtained under these operating conditions as compared to an engine equipped with a valve in which the timing is dependent on the engine's appetite for an intake charge.